Pipelined SAR with TDC converter

ABSTRACT

An analog-to-digital converter (ADC) is disclosed. The ADC includes a successive approximation register and a voltage-to-time conversion element. The successive approximation register is configured to receive an input signal and to generate a first digital signal and a residue voltage. The voltage-to-time conversion element is configured to convert the residue voltage to a time domain representation. The voltage-to-time conversion element includes an amplifier having an input coupled to an output of the successive approximation register and configured to receive the residue voltage, and a zero crossing detector directly coupled to an output of the amplifier. A time-to-digital converter is coupled to an output of the zero crossing detector and is configured to generate a second digital signal.

REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.15/903,711, filed on Feb. 23, 2018, which is a Continuation of U.S.application Ser. No. 15/590,247, filed on May 9, 2017 (now U.S. Pat. No.9,923,568, issued on Mar. 20, 2018), which is a Continuation of U.S.application Ser. No. 14/959,064, filed on Dec. 4, 2015 (now U.S. Pat.No. 9,673,835, issued on Jun. 6, 2017). The contents of theabove-referenced patent applications are hereby incorporated byreference in their entirety.

BACKGROUND

An analog-to-digital converter (ADC) is a device that converts an analoginput signal to a digital output signal comprising a sequence of bitsthat represent an amplitude of the analog input signal. An ADC typicallyhas an analog reference voltage or current against which the analoginput signal is compared. The digital output signal indicates whatfraction of the reference voltage is the amplitude of the analog inputsignal. Analog-to-digital converters (ADCs) are widely used by modernday digital electronic systems to convert analog signals to digitalsignals that can be used by the digital electronic systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a block diagram of some embodiments of a hybridsuccessive-approximation-register (SAR) analog-to-digital converter(ADC).

FIG. 2 illustrates some additional embodiments of a hybrid SAR-ADC.

FIG. 3 shows some embodiments of timing diagrams illustrating operationof the hybrid SAR-ADC of FIG. 2.

FIG. 4 illustrates a more detailed embodiment of a voltage-based signalprocessing element within a hybrid SAR-ADC.

FIG. 5 illustrates a more detailed embodiment of a time-based signalprocessing element within a hybrid SAR-ADC.

FIGS. 6A-6B illustrate a more detailed embodiment of a voltage-to-timeconversion element within a disclosed hybrid SAR-ADC.

FIG. 7 illustrates a flow diagram of some embodiments of a method ofperforming an analog-to-digital conversion using voltage-based signalprocessing and time-based signal processing.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Successive-approximation-register analog-to-digital converters(SAR-ADCs) are a type of analog-to-digital converter commonly used inintegrated chips. An SAR-ADC converts an analog input signal into adigital output signal having a plurality of bits via a binary searchalgorithm that converges on the analog input signal. Typically, SAR-ADCsmay operate by way of a voltage-based signal processing. Involtage-based signal processing, the analog input signal is converted toa voltage, which is compared to a reference voltage to determine a firstbit. After determining a first bit, the SAR-ADC moves to a next bit andperforms another comparison to an updated reference voltage generatedbased upon the first bit. The sequence of comparisons continues until ann-bit digital word is generated.

As the scaling of integrated chips continues, the power efficiency oftraditional ADCs using voltage-based signal processing decreases. Thisis because such ADCs (e.g., Pipelined ADCs) use high gain amplifiersthat consume large amounts of power and which are not able to be readilyscaled to emerging technology nodes (e.g., to a technology node having aminimum feature size of 16 nm, 10 nm, etc.).

The present disclosure relates to a hybrid SAR-ADC that uses acombination of voltage-based signal processing and time-based signalprocessing to convert an analog input signal to a digital output signal,and an associated method. In some embodiments, the hybrid SAR-ADCcomprises a voltage-based signal processing element configured toconvert an analog input signal to a first digital signal having aplurality of most significant bits (MSBs) and to generate a residuevoltage from an input voltage and the first digital signal. Avoltage-to-time conversion element is configured to convert the residuevoltage to a time domain representation. A time-based signal processingelement is configured to convert the time domain representation to asecond digital signal comprising a plurality of least significant bits(LSBs). By determining the plurality of MSBs using voltage-based signalprocessing and determining the plurality of LSBs using time-based signalprocessing, the hybrid SAR-ADC is able to achieve a low power and acompact area.

FIG. 1 illustrates a block diagram of some embodiments of a hybridsuccessive-approximation-register analog-to-digital converter (SAR-ADC)100.

The hybrid SAR-ADC 100 comprises a voltage-based signal processingelement (SPE) 102 and a time-based signal processing element (SPE) 104,which are collectively configured to convert an analog input signalS_(IN) to a digital output signal D_(out). The voltage-based signalprocessing element 102 is configured to generate a first digital signald_(MSB) corresponding to a plurality of most significant bits (MSB),while the time-based signal processing element 104 is configured togenerate a second digital signal d_(LSB) corresponding to a plurality ofleast significant bits (LSB).

In some embodiments, the voltage-based signal processing element 102comprises a successive-approximation-register (SAR) block 108. The SARblock 108 is configured to implement a binary search algorithm todetermine digital values of the first digital signal d_(MSB). Thedigital values correspond to the analog input signal S_(IN) for theplurality of most significant bits (MSB). The SAR block 108 is alsoconfigured to generate a residue voltage V_(RES). The residue voltageV_(RES) corresponds to a difference between an input voltage determinedfrom the analog input signal and a voltage corresponding to the firstdigital signal d_(MSB).

As the digital values for the first digital signal d_(MSB) aredetermined, the voltage converges on the analog input signal S_(IN), andthe residue voltage V_(RES) (i.e., the difference between the inputvoltage and the reference voltage) decreases. The VTC element 110 isconfigured to generate a time domain representation T_(DIS) of theresidue voltage V_(RES) by performing a voltage-to-time conversion ofthe residue voltage V_(RES) output from the SAR block 108. The timedomain representation T_(DIS) comprises a pulse having a width thatcorrespond to a value of the residue voltage V_(RES). For example, insome embodiments, a larger residue voltage V_(RES) may cause the timedomain representation T_(DIS) to have a pulse with a greater pulse widththan a smaller residue voltage.

The time domain representation T_(DIS) is provided to the time-basedsignal processing element 104. In some embodiments, the time-basedsignal processing element 104 may comprise a time-to-digital converter112 configured to convert the time domain representation T_(DIS) to asecond digital signal d_(LSB). The second digital signal d_(LSB) hasdigital values for the plurality of least significant bits (LSB).

The first digital signal d_(MSB) and second digital signal d_(LSB) areprovided to a digital output signal generation element 106. The digitaloutput signal generation element 106 is configured to combine the firstdigital signal d_(MSB) and the second digital signal d_(LSB) to generatea digital output signal D_(out). By employing a combination ofvoltage-based signal processing and time-based signal processing, thehybrid SAR-ADC 100 is able to achieve a low power and a compact area.This is because the time-based signal processing is able to work at lowvoltages that allow for the overall power of the hybrid SAR-ADC 100 tobe low. Furthermore, the hybrid SAR-ADC 100 offers a resolution thatimproves with deep submicron scaling (e.g., to a technology node havinga minimum feature size of 16, 10 nm, etc.), so as to provide the hybridSAR-ADC 100 with scalability benefits over traditional SAR-ADCs.

FIG. 2 illustrates some additional embodiments of a hybridsuccessive-approximation-register analog-to-digital converter (SAR-ADC)200.

The hybrid SAR-ADC 200 comprises a SAR block 202 configured to generatea first digital signal d_(MSB) corresponding to a plurality of mostsignificant bits (MSBs) of an analog input signal S_(IN). The SAR block202 comprises a track-and-hold circuit 204 coupled to an input nodeconfigured to receive the analog input signal S_(IN). The track-and-holdcircuit 204 is configured to sample (i.e., capture) a value of theanalog input signal S_(IN) and to hold the sampled value at a constantlevel for a period of time. The track-and-hold circuit 204 is configuredto output the sampled value as an input voltage V_(IN) to an arithmeticunit 206. The arithmetic unit 206 is configured to determine a residuevoltage V_(RES), which is equal to a difference of the input voltageV_(IN) and a DAC voltage V_(DAC) output from a capacitivedigital-to-analog converter (CDAC) 212.

The residue voltage V_(RES) is provided from the arithmetic unit 206 toa comparator 208. The comparator 208 is configured to make a comparisonto determine if the input voltage V_(IN) is greater than or less thanthe DAC voltage V_(DAC). For example, the comparator 208 is configuredto generate a comparator signal S_(comp) based upon a comparison of theresidue voltage V_(RES) and a ground potential GND. The comparatorsignal S_(comp) is used to determine a value of a next bit of the firstdigital signal d_(MSB). For example, if the input voltage V_(IN) isgreater than the DAC voltage V_(DAC), the comparator signal S_(comp)will cause a next bit of the first digital signal d_(MSB) to have afirst value (e.g., a “1”), while if the input voltage V_(IN) is lessthan the DAC voltage V_(DAC) the comparator signal S_(comp) will causethe next bit of the first digital signal d_(MSB) to have a differentsecond value (e.g., a “0”).

The comparator signal S_(comp) is provided to a SAR logic unit 210. TheSAR logic unit 210 is configured to generate control bits based upon theoutput of the comparator 208.

The CDAC 212 is configured to generate a new DAC voltage V_(DAC) basedupon the control bits, which is used to determine a next bit of thefirst digital signal d_(MSB). In some embodiments, the control bits fromsuccessive iterations of a binary search algorithm implemented by theSAR block 202 may be stored in a register 211 within the SAR logic unit210 and be provided as the first digital signal d_(MSB).

For example, to implement the binary search algorithm, the first digitalsignal d_(MSB) may be initially set to a value of “100000” (i.e., tohave a most significant bit set to “1”). This value may cause the DACvoltage V_(DAC) to be equal to V_(REF)/2, where V_(REF) is a referencevoltage provided to the CDAC 212. The arithmetic unit 206 determines aresidue voltage V_(RES) from a difference between the DAC voltageV_(DAC) and the input voltage V_(IN), and the comparator 208 performs acomparison to determine if the input voltage V_(IN) is greater than orless than the DAC voltage V_(DAC). If the input voltage V_(IN) isgreater than the DAC voltage V_(DAC), the comparator 208 outputs a highlogical state (i.e., a “1”) and the MSB of the first digital signald_(MSB) remains at 1. Conversely, if the input voltage V_(IN) is lessthan the DAC voltage V_(DAC), the comparator 208 outputs a low logicalstate (i.e., a “0”) and the MSB of the first digital signal d_(MSB) ischanged to the logical low state. The SAR logic unit 210 then moves to anext bit and performs another comparison. The sequence continues for anumber of bits until the first digital signal d_(MSB) comprises an n-bitdigital word.

The residue voltage V_(RES) is also provided to a voltage-to-timeconversion (VTC) element 214. The VTC element 214 is configured toperform a voltage-to-time conversion of the residue voltage V_(RES) to atime domain representation T_(DIS). The VTC element 214 comprises acurrent source element I_(DIS) 215 which is configured to discharge thereside voltage V_(RES) from the SAR block 202. The time taken todischarge the residue voltage V_(RES) is directly proportional to thevalue of the residue voltage V_(RES). In some embodiments, a residueoffset circuit 217 is configured to provide a offset voltage V_(OS) tothe arithmetic unit 206. The offset voltage V_(OS) is configured tocause the residue voltage V_(RES) to discharge in one directionregardless of whether a sign of the residue voltage V_(RES), so that theoutput of the VTC element 214 is linear.

In some embodiments, the VTC element 214 comprises an amplifier 218 thatis configured to amplify the residue voltage V_(RES). The amplifiedresidue voltage V_(RES)′ is then provided to a zero crossing detector220. The zero crossing detector 220 is configured to convert theamplified residue V_(RES)′ to a time domain representation T_(DIS). Insome embodiments, the zero crossing detector 220 may comprise acomparator configured to generate the time domain representation T_(DIS)by comparing the amplified residue voltage V_(RES)′ to a groundpotential GND, so that the zero crossing detector 220 produces an outputstate change whenever the amplified residue V_(RES)′ crosses the groundpotential GND. In some embodiments, the amplifier 218 may comprise anopen loop amplifier. The use of an open loop amplifier minimizes powerconsumption of the voltage-to-time conversion (VTC) element 214.

The time domain representation T_(DIS) is provided to a time-to-digitalconversion element 221 configured to convert the time domainrepresentation T_(DIS) to the second digital signal d_(LSB). In someembodiments, the time-to-digital conversion element 221 may comprise apulse generator 222 configured to receive the time domain representationT_(DIS) and to generate pulsed stop and start signals that are providedto a time-to-digital converter (TDC) 224. The pulsed stop and startsignals are offset in time by an amount that is proportional to the timedomain representation T_(DIS). The TDC 224 is configured to convert thepulsed stop and start signals to a second digital signal d_(LSB)corresponding to a plurality of least significant bits (LSBs) of theanalog input signal S_(IN).

The first digital signal d_(MSB) and the second digital signal d_(LSB)are provided to a digital error correction element 226 configured togenerate a digital output signal D_(out) therefrom. In variousembodiments, the first digital signal d_(MSB) and/or the second digitalsignal d_(LSB) may have any number of bits. In some embodiments, thefirst digital signal d_(MSB) and/or the second digital signal d_(LSB)may have redundant bits, which can be used by the digital errorcorrection element 226 to determined errors in the hybrid SAR-ADC 200.For example, the first digital signal d_(MSB) may comprise an 8-bitsignal and the second digital signal d_(LSB) may comprise a 6-bitsignal, which are combined by the digital error correction element 226to generate a 12-bit digital output signal D_(out). In some embodiments,the digital error correction element 226 may comprise one or more shiftregisters configured to time-align bits from the first digital signald_(MSB) and the second digital signal d_(LSB) prior to providing thebits to the digital-error-correction logic.

FIG. 3 shows some embodiments of timing diagrams 300 illustratingoperation of the hybrid SAR-ADC 200 of FIG. 2.

Graph 302 shows the DAC voltage V_(DAC) and the input voltage V_(IN)(shown along the y-axis) as a function of time (shown along the x-axis).Graph 304 shows the residue voltage V_(RES) as a function of time. Theresidue voltage V_(RES) is equal to a difference of the DAC voltageV_(DAC) and the input voltage V_(IN) (i.e., V_(RES)=V_(DAC)−V_(IN)). Asshown in graph 304, as the SAR block 202 determines more bits of thefirst digital signal d_(MSB), the residue voltage V_(RES) approacheszero (i.e., the DAC voltage V_(DAC) converges upon the input voltageV_(IN)).

Graph 306 illustrates the first digital signal d_(MSB) (i.e., thecontrol bits of the SAR logic element). Initially, the first digitalsignal d_(MSB) is set to “100000.” During a first time period from t₀ tot₁ a first comparison is performed. Since the residue voltage V_(RES) isgreater than 0 (i.e., the input voltage V_(IN) is greater than the DACvoltage V_(DAC)), the MSB remains at 1, as shown in graph 306. During asecond time period, from t₁ to t₂, a second comparison is performed.Since the residue voltage V_(RES) is less than 0 (i.e., the inputvoltage V_(IN) is less than the DAV voltage V_(DAC)), a second bitremains at 0, so that the DAC is set to “100000.” During a third timeperiod, from t₂ to t₃, a third comparison is performed. Since theresidue voltage V_(RES) is greater than 0, a third bit is changed to“1”, so that the control bits of the SAR logic element are set to“101000.” Successive comparisons result in a first digital signald_(MSB) having a value of “101000.”

Graph 308 illustrates a time domain representation T_(DIS) of theresidue voltage V_(RES). The time domain representation T_(DIS) has apulse comprising a width w that is dependent upon the residue voltageV_(RES). A second digital signal d_(LSB) is generated from the timedomain representation T_(DIS). Since the time domain representationT_(DIS) corresponds to a residue of the SAR block 202, the seconddigital signal d_(LSB) corresponds to a LSB of the analog input signal.

FIG. 4 illustrates a more detailed embodiment of a hybridsuccessive-approximation-register analog-to-digital converter (SAR-ADC)400.

The hybrid SAR-ADC 400 comprises a SAR block 401 and a VTC element 214.The SAR block 401 is configured to generate a first digital signald_(MSB) corresponding to a plurality of most significant bits (MSB) ofan input voltage V_(IN). The VTC element 214 is configured to receive aresidue voltage V_(RES) from the SAR block 401 and to generate a timedomain representation T_(DIS) of the residue voltage V_(RES). In someembodiments, a residue offset circuit 217 configured to generate anoffset voltage V_(OS) is selectively coupled to the VTC element 214 byway of a switching element 406.

The SAR block 401 comprises a capacitive digital-to-analog converter(CDAC) 402 connected to an output of SAR logic unit 210. The CDAC 402comprises a plurality of capacitors 404 coupled between a commonterminal 403 and free terminals. The plurality of capacitors 404 havebinary weighted values. The common terminal 403 is selectively coupledto a ground potential GND by way of a first switching element S₁. Thefree terminals are selectively coupled to the input voltage V_(IN)and/or one or more reference voltages V_(REFM) and V_(REFP) by way ofadditional switching elements SB.

During an acquisition phase, the common terminal 403 is held at theground potential GND and the free terminals are connected to an inputvoltage V_(IN). After the acquisition phase, the common terminal 403 isdisconnected from the ground potential GND and the free terminals aredisconnected from the input voltage V_(IN), causing the plurality ofcapacitors 404 to hold a charge that is proportional to the inputvoltage V_(IN). The free terminals of the plurality of capacitors 404are then connected to a minimum reference voltage V_(REFM) (e.g.,ground), driving the common terminal 403 to a voltage equal to −V_(IN).The MSB capacitor is then disconnected from the minimum referencevoltage V_(REFM) and connected to a maximum reference voltage V_(REFP),driving the common terminal 403 in the positive direction by an amountequal to ½V_(REF) so that V_(DAC)=−V_(IN)+½V_(REF). The comparator 208is configured to output a 1 if V_(RES)<0 or a 0 if V_(RES)>0. If thecomparator 208 outputs a 1, then the MSB capacitor 404 a stays connectedto a maximum reference voltage V_(REFP). Otherwise, the MSB capacitor404 a is connected back to the minimum reference voltage V_(REFM) (e.g.,ground). The next smaller capacitor 404 b is then connected to theminimum reference voltage V_(REFM) and the new DAC voltage V_(DAC) iscompared with the minimum reference voltage V_(REFM). This processcontinues until the first digital signal d_(MSB) has been determined.

FIG. 5 illustrates a more detailed embodiment of a time-based signalprocessing element 502 within a hybrid SAR-ADC 500.

The time-based signal processing element 502 is configured to receive atime domain representation T_(DIS) of a residue voltage V_(RES) from avoltage-to-time conversion (VTC) element 214. In some embodiments, thetime-based signal processing element 502 may comprise a time-to-digitalconverter (TDC). The time-based signal processing element 502 comprisesa tapped delay line 506 comprising a plurality of delay elements 506a-506 n arranged in series to a pulse generator 504. The plurality ofdelay elements 506 a-506 n are configured to introduce a delay into astart signal S_(d0) generated by the pulse generator 504 to generate aplurality of delayed signals S_(d1)-S_(dn).

The plurality of delay elements 506 a-506 n respectively have an outputcoupled to a D-input of a corresponding flip-flop 508 a-508 n. Theflip-flops 508 a-508 n further comprise a clock input. In someembodiments, the clock input is coupled to the time domainrepresentation T_(DIS), which acts as a stop signal. In otherembodiments, the clock input may be coupled to a pulse generator (notshown) configured to generate a stop signal that is delayed with respectto the start signal S_(d0) by an amount that is proportional to the timedomain representation T_(DIS) of the residue voltage V_(RES).

The state of the tapped delay line 506 is sampled at a time of arrivalof the time domain representation T_(DIS), such that a rising/fallingedge of the time domain representation T_(DIS) (i.e., the stop signal)latches the status of the tapped delay line 506. In some embodiments,the output of the plurality of flip flops 508 a-508 n may be provided toa thermometer code converter 510 configured to generate the seconddigital output signal d_(LSB).

Since the VTC element 214 has represented the residue voltage V_(RES) asa time domain representation T_(DIS) in which the zero crossing point isindicated by a rising/falling edge, the time-based signal processingelement 502 determines a values of the second digital signal d_(LSB)from when the rising/falling edge occurs (i.e., that indicates a digitalvalue of the residue voltage V_(RES) using time domain signalprocessing). In other words, a time interval between start signal S_(d0)and a rising/falling edge of the time domain representation T_(DIS) isproportional to the number of flip-flops that were sampled astransparent, so that the outputs of the flip-flops 508 a-508 n define asecond digital signal d_(LSB) having bits with values that depend uponthe residue voltage V_(RES) (e.g., a first time difference may providefor a second digital signal d_(LSB) having a first sequence of bits(e.g., “1000”) while a second time difference may provide for a seconddigital signal d_(LSB) having a second sequence of bits (e.g., “1110”)).

FIGS. 6A-6B illustrates a more detailed embodiment of a voltage-to-timeconversion element 602 within a hybrid SAR-ADC 600.

The hybrid SAR-ADC 600 comprises a SAR block 401 comprising a SAR logicunit 210 configured to generate a DAC control signal Φ_(DAC) having aplurality of bits. The DAC control signal Φ_(DAC) is provided to theCDAC 402 to control biasing of a plurality of capacitors 404 within theCDAC 402. Based upon the DAC control signal Φ_(DAC), the CDAC 402 willoutput a DAC voltage V_(DAC). The DAC voltage V_(DAC) is provided to anarithmetic unit 206 that is configured to subtract the DAC voltageV_(DAC) from an input voltage V_(IN) to generate a residue voltageV_(RES). The residue voltage V_(RES) is provided to a voltage-to-timeconversion (VTC) element 602.

The VTC element 602 comprises an amplifier 218 connected to thearithmetic unit 206 and a zero crossing detector 220 connected to theamplifier 218. The amplifier 218 is further connected to a dischargecurrent source 606 configured to generate a discharge current I_(DIS)that discharges the plurality of capacitors 404 within the CDAC 402. Thetime it takes to discharge the plurality of capacitors 404 isproportional to the residue voltage V_(RES). Therefore, when the zerocrossing point of the residue voltage V_(RES) is reached, the zerocrossing detector 220 will generate a time domain representation T_(DIS)comprising a pulse having a width that is a proportional to the residuevoltage V_(RES). The time domain representation T_(DIS) may besubsequently converted to a second digital signal comprising theplurality of least significant bits.

In some embodiments, a residue offset circuit 604 is connected to anamplifier 218. The residue offset circuit 604 is configured to cause theresidue voltage V_(RES) to discharge in one direction (e.g., todischarge from a positive value to zero or to discharge from a negativevalue to zero), regardless of whether the residue voltage V_(RES) ispositive or negative. By causing the residue voltage V_(RES) todischarge in one direction, the time domain representation T_(DIS) islinear regardless of the characteristics of the amplifier 218.

The residue offset circuit 604 comprises a COS capacitive element 608.The COS capacitive element 608 is selectively coupled to a groundpotential GND and to an offset voltage source V_(OS) by way of switchingelements S₃-S₅. The COS capacitive element 608 is also coupled to thedischarge current source 606 by way of switching element S₆. When thedischarge current source 606 generates the discharge current I_(DIS),the COS capacitive element 608 also discharges to provide a residueoffset voltage V_(OS) to the amplifier 218.

FIG. 6B illustrates timing diagrams 610 illustrating operation of thehybrid SAR-ADC 600 of FIG. 6A.

Graph 612 illustrates an input signal Φ_(TH) corresponding to the inputvoltage V_(IN) output from the track-and-hold circuit 204. The inputsignal Φ_(TH) is held at a value of an analog input signal for a periodof time. For example, at time t₁ the input signal Φ_(TH) is held at afirst value V_(INI) for a time period that extends from time t₁ to timet₂. At other times the input signal Φ_(TH) is held low. At a first timet1, when the input signal Φ_(TH) is high, switching elements S₁-S₄ areclosed, thereby initializing (e.g., connecting to GND) the capacitorswithin the CDAC 402 and also resetting the capacitance value of the COScapacitive element 608. At a second time t₂, when the input signalΦ_(TH) is low, switching elements S₁-S₄ are open, thereby disconnectingthe capacitors from the ground potential GND.

Graph 614 illustrates DAC control signal Φ_(DAC) provided to the CDAC402 to control biasing of a plurality of capacitors 404 within the CDAC402. The DAC control signal is configured to generate a plurality ofpulses Φ_(DAC,1)-Φ_(DAC,4), which sequentially bias the plurality ofcapacitors 404 in the CDAC 402 by operating switching blocks SB₁-SB₄ toconnect the plurality of capacitors 404 to an input voltage and/or oneor more reference voltages as described above.

Graph 616 illustrates a control bit signal Φ_(BIT) that operatesswitching element S₇ to selectively couple the comparator 208 to aground potential GND. The comparator 208 performs a comparison on afalling edge of the control bit signal Φ_(BI).

Graph 618 illustrates a residue voltage discharge signal Φ_(DIS) thatoperates switching element S₆ to selectively couple the dischargecurrent source 606 to CDAC 402 and to COS capacitive element 608. Forexample, at a time t3, when the residue voltage V_(RES) is generatedfrom the first digital signal d_(MSB), the residue voltage dischargesignal Φ_(DIS) is set to high to couple the discharge current source 606to the CDAC 402 and to the COS capacitive element 608. The dischargecurrent causes charges from the plurality of capacitors 404 within theCDAC 402 and the COS capacitive element 608 to start discharging in alinear fashion. Once the plurality of capacitors 404 within the CDAC 402have discharged and the residue voltage V_(RES) is equal to the groundpotential GND, the amplifier 218 outputs a signal that causes the zerocrossing detector 220 to generate a pulse that is proportional to theresidue voltage V_(RES).

Graph 620 illustrates an amplifier control signal Φ_(A). The amplifiercontrol signal Φ_(A) operates switching element S₈ to selectively couplethe amplifier 218 to the SAR block 401 and the residue offset circuit604. For example, at time t₄, the amplifier control signal Φ_(A) is setto high to close switching element S₈, so that a sum of the residuevoltage V_(RES) and the residue offset voltage V_(OS) is provided to theamplifier 218. The resulting output voltage Vo of the amplifier is equalto Vo=[(V_(IN)−V_(DAC))+V_(OS)−I_(DIS)*T_(DIS)/C_(T)]*A_(OL), so thatthe zero-crossing of the output voltage Vo is detected atT_(DIS)=(V_(IN)−V_(DAC)+V_(OS))*C_(T)/I_(DIS) (where C_(T) is equal tothe sum of the capacitors 404). Therefore, time domain representationT_(DIS) at zero crossing is linear regardless of the characteristics ofthe amplifier 218, so that the amplifier 218 may comprise a low poweropen loop amplifier with a moderately linear low gain.

Graph 622 illustrates an amplifier reset signal Φ_(CM). The amplifierreset signal Φ_(CM) operates switching element S₉ to selectively couplethe amplifier 218 to a ground potential GND. For example, at time t₅,the amplifier reset signal Φ_(CM) is set to high to close switch S₉ sothat the value of the time domain representation T_(DIS) goes low.

FIG. 7 illustrates a flow diagram of some embodiments of a method 700 ofperforming an analog-to-digital conversion using voltage-based signalprocessing and time-based signal processing.

While the disclosed method 700 is illustrated and described herein as aseries of acts or events, it will be appreciated that the illustratedordering of such acts or events are not to be interpreted in a limitingsense. For example, some acts may occur in different orders and/orconcurrently with other acts or events apart from those illustratedand/or described herein. In addition, not all illustrated acts may berequired to implement one or more aspects or embodiments of thedescription herein. Further, one or more of the acts depicted herein maybe carried out in one or more separate acts and/or phases.

At operation 702, an analog input signal is sampled to determine aninput voltage.

At operation 704, a residue voltage is determined based upon adifference of an input voltage and a DAC voltage determined from thefirst digital signal.

At operation 706, a first digital signal comprising a plurality of mostsignificant bits (MSBs) is determined from the residue voltage. In someembodiments, acts 702-706 may be repeated multiple times (as shown byarrow 708) to determine a plurality of bits for the first digitalsignal.

At operation 710 a residue offset voltage is added to the residuevoltage, in some embodiments.

At operation 712, the residue voltage is converted to a time domainrepresentation of the residue voltage.

At operation 714, the time domain representation is converted to asecond digital signal comprising a plurality of least significant bits(LSBs).

At operation 716, the first digital signal and the second digital signalare combined to generate a digital output signal.

Therefore, the present disclosure relates to an analog-to-digitalconverter (ADC) that uses a combination of voltage-based signalprocessing and time-based signal processing to convert an analog inputsignal to a digital output signal, and an associated method offormation.

In some embodiments, the present disclosure relates to an analog todigital converter (ADC). The ADC includes a voltage-based signalprocessing element configured to receive an input signal and to generatea first digital signal having a plurality of most significant bits and aresidue voltage. A residue offset circuit is configured to provide aresidue offset voltage to the residue voltage, and a voltage-to-timeconversion element is configured to convert a sum of the residue voltageand the residue offset voltage to a time domain representation. Atime-based signal processing element is configured to convert the timedomain representation to a second digital signal having a plurality ofleast significant bits.

In other embodiments, the present disclosure relates to an analog todigital converter (ADC). The ADC includes a voltage-based signalprocessing element configured to receive an input signal and to generatea first digital signal having a first number of most significant bitsand a residue voltage. A voltage-to-time conversion element isconfigured to convert the residue voltage to a time domainrepresentation, and a time-based signal processing element is configuredto convert the time domain representation to a second digital signalcomprising a second number of least significant bits. A digital errorcorrection element is configured to receive the first digital signal andthe second digital signal and to generate a digital output signaltherefrom. The digital output signal has a third number of bits that isless than a sum of the first number and the second number.

In yet other embodiments, a method of performing an analog-to-digitalconversion. The method includes generating a first digital signal havinga first number of most significant bits based upon an analog inputsignal, and determining a residue voltage based upon the first digitalsignal. The method further includes adding a residue offset voltage tothe residue voltage, and converting a sum of the residue voltage and theresidue offset voltage to a time domain representation. The methodfurther includes converting the time domain representation to a seconddigital signal having a second number of least significant bits.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An analog-to-digital converter (ADC), comprising:a successive approximation register configured to receive an inputsignal and to generate a first digital signal and a residue voltage; avoltage-to-time conversion element configured to convert the residuevoltage to a time domain representation, the voltage-to-time conversionelement comprising: an amplifier comprising an input coupled to anoutput of the successive approximation register; a zero crossingdetector directly coupled to an output of the amplifier; and atime-to-digital converter coupled to an output of the zero crossingdetector and configured to generate a second digital signal.
 2. The ADCof claim 1, wherein the successive approximation register comprises: anarithmetic unit having a first input and a second input; a comparatorcoupled to an output of the arithmetic unit; a successive approximationregister (SAR) logic unit coupled to an output of the comparator; acapacitive digital-to-analog converter (CDAC) coupled to an output ofthe SAR logic unit and to the first input of the arithmetic unit; and atrack-and-hold circuit having an input configured to receive the inputsignal and an output coupled to the second input of the arithmetic unit.3. The ADC of claim 2, wherein the track-and-hold circuit is configuredto provide a voltage to the second input of the arithmetic unit, thevoltage having a value that corresponds to a value of the input signalat a first time; and wherein the arithmetic unit is configured togenerate the residue voltage based upon the voltage and a signal outputfrom the CDAC.
 4. The ADC of claim 2, wherein the CDAC comprises aplurality of capacitors; and wherein the time domain representation isproportional to a sum of capacitances of the plurality of capacitors. 5.The ADC of claim 1, wherein the zero crossing detector comprises acomparator configured to generate the time domain representation bycomparing a voltage that is output from the amplifier to a groundpotential.
 6. The ADC of claim 1, wherein the time-to-digital convertercomprises: a plurality of delay elements arranged in series; a pluralityof flip-flops, wherein the plurality of delay elements respectively havean output coupled to a first input of a corresponding one of theplurality of flip-flops and wherein the plurality of flip-flops furthercomprise a second input that is coupled to the output of the zerocrossing detector; and a thermometer code converter coupled to outputsof the plurality of flip-flops, wherein the thermometer code converteris configured to generate the second digital signal.
 7. The ADC of claim1, wherein the amplifier comprises an open loop amplifier configured toprovide the voltage-to-time conversion element with a low powerconsumption.
 8. The ADC of claim 1, further comprising: a digital errorcorrection circuit configured to receive the first digital signal andthe second digital signal and to output a digital output signal.
 9. TheADC of claim 1, further comprising: a residue offset circuit coupled tothe input of the amplifier, wherein the residue offset circuit isconfigured to provide a residue offset voltage that causes thevoltage-to-time conversion element to generate the time domainrepresentation as a linear function.
 10. An analog-to-digital converter(ADC), comprising: a successive-approximation-register block configuredto receive an input signal and to generate a first digital signal and aresidue voltage; a residue offset circuit configured to generate aresidue offset voltage; a conversion element comprising an amplifierthat is configured to output an output voltage that is a function of theresidue voltage and the residue offset voltage and a comparatorconfigured to convert the output voltage to a time domain representationof the residue voltage; and a time-to-digital converter configured togenerate a second digital signal from the time domain representation.11. The ADC of claim 10, wherein the time domain representation that isoutput from the conversion element is a linear function.
 12. The ADC ofclaim 10, further comprising: a discharge current source operablycoupled to an input of the amplifier and an output of thesuccessive-approximation-register block, wherein the discharge currentsource is configured to generate a discharge current; and a capacitiveelement selectively coupled to a ground potential by way of a firstswitching element, to an offset voltage source by way of a secondswitching element, and to the discharge current source by way of a thirdswitching element.
 13. The ADC of claim 12, further comprising: a fourthswitching element disposed between the discharge current source and theinput of the amplifier.
 14. The ADC of claim 12, wherein the outputvoltage generated by the amplifier is proportional to a product of thedischarge current and a gain of the amplifier.
 15. A method ofperforming an analog to digital conversion, comprising: operating asuccessive approximation register to generate a first digital signal anda residue voltage from an analog input signal; operating avoltage-to-time conversion element to generate a time domainrepresentation based upon the residue voltage and a residue offsetvoltage; and operating a time-to-digital converter to generate a seconddigital signal based upon the time domain representation.
 16. The methodof claim 15, further comprising: generating a discharge current that isconfigured to cause the residue voltage to discharge from the successiveapproximation register.
 17. The method of claim 15, wherein the timedomain representation is proportional to the residue offset voltage. 18.The method of claim 15, wherein the time domain representation is alinear function.
 19. The method of claim 15, further comprising:operating a first switching element to couple a discharge current from adischarge current source to an output of the successive approximationregister at a first time; and operating a second switching element tocouple the output of the successive approximation register to an inputof the voltage-to-time conversion element at a second time after thefirst time.
 20. The method of claim 19, further comprising: operatingthe first switching element to decouple the discharge current from theoutput of the successive approximation register at a third time afterthe second time; and operating the second switching element to decouplethe output of the successive approximation register from the input ofthe voltage-to-time conversion element at the third time.